1. Field of the Invention
This invention relates to an adjustable jack and more particularly to an adjustable mine roof support jack having an adjusting screw operable in one mode for rapid nonrotational linear advancement to the desired working height and in a second mode for slow rotational advancement to a final load bearing position.
2. Description of the Prior Art
In underground mining, as the working face is advanced by the mining machine the entryway thus formed has an unsupported roof which must be supported by a roof control system to prevent roof subsidence. In the past it was the conventional practice to provide roof control by building cribs, setting timbers and the like. However, such roof control measures involve a large amount of labor and substantial expense in supplying materials which are not recoverable for reuse.
More recently mechanical roof control systems have been developed to provide rapidly adjustable means for supporting a mine roof, such as hydraulic jacks, rams, and props. However, these devices are complex requiring a large inventory of replacement parts and are subject to malfunction which delay the erection of the roof support system and consequently the mining operation.
A known means for providing a dependable roof support system that is quick setting and sufficiently rugged in construction for use in a mine is the screw-type roof jack which is adjustable in height to provide roof control in any underground mine. Examples of such adjustable jacks are disclosed in U.S. Pat. Nos. 1,784,991 and 1,785,069 and include a main tubular body portion having a telescoping screw rod that is releasably engagable with an adjustment means carried by the tubular body portion. To effect rapid extension of the screw rod into load bearing relation with the mine roof, the adjustment means is released from threaded engagement with the screw rod to permit the screw rod to extend without requiring lengthy revolving of a crank handle. In this manner the jack is quickly extensible to the desired load bearing height as determined by the height of the mine roof. Final load bearing adjustment is accomplished by returning the adjusting means to threaded engagement with the screw rod for rotational advancement thereof by turning the adjusting means. A similar device is disclosed in U.S. Pat. No. 3,210,047 for shoring concrete slabs to various lengths in which a shoring jack includes jaws movable into and out of clamping engagement with a load engaging tube to facilitate adjustments in the shoring length.
In U.S. Pat. No. 3,603,552 an adjustable strut is provided with an inner screw thread releasably engagable by a rotatable, split nut which when disengaged from the screw thread effects rapid adjustment of the strut. The ends of the strut are provided with load-engaging thrust plates, and the overall length of the strut between the thrust plates is adjusted by rotating the nut when engaged with the screw thread. U.S. Pat. Nos. 3,022,043 and 3,667,730 disclose quick acting trailer jacks for supporting and leveling a travel trailer. The jacks are permanently mounted to the draw bar of the trailer and are operable for rapid extension and retraction without requiring lengthy revolving of a crank handle.
Mechanical jacks featuring quickly adjustable extensions are also disclosed in U.S. Pat. Nos. 1,612,124; 2,218,319; and 2,316,432. Each of these devices includes clamping means for releasably engaging an elongated threaded rod to permit telescoping action of the jack by both rapid nonrotational advancement and slower rotational advancement of the jack.
A hingedly connected split nut is also illustrated in U.S. Pat. No. 2,654,567. The split nut includes halves which are internally threaded to engage the external threads of a tubular screw. A pipe to be pulled extends through the hollow interior of the tubular screw. Rotation of a ratchet pulls the pipe in a desired direction.
While it has been suggested by the prior art devices to provide adjustable jacks operable for rapid nonrotational linear extension and for rotational extension to a final setting, there is need for an adjustable jack that is operable to be quickly converted between nonrotational and rotational extension to facilitate quick setting of the jack as in use for mine roof control and to maintain a compressive load upon the mine roof without slippage of the telescoping member.